FIG. 2 shows an example of a TFT substrate for a liquid crystal display panel, using an amorphous silicon (hereinafter simply referred to as a-Si) TFT, using Al as a gate electrode and constituting a portion of a gate insulation film with Al.sub.2 O.sub.3 obtained by anodic oxidation thereof. FIGS. 2(a), (b) and (c) show, respectively, an equivalent circuit diagram, a plan view and a cross sectional view of the TFT substrate. There are depicted for gate terminals by G.sub.1, G.sub.2, gate bus-lines by G.sub.1 ', G.sub.2 ', drain terminals by D.sub.1, D.sub.2, TFT by T.sub.11, T.sub.12, T.sub.21, T.sub.22, liquid crystal by LC, common terminal disposed on the side of the color filter substrate by V.sub.com. Further, there are also depicted substrate 10, Al 12', Al.sub.2 O.sub.3 13, SiN 14, transparent electrode (pixel electrode) 17, non-doped a-Si(i) 15, phosphorus-doped hydrogenated amorphous silicon (hereinafter simply referred to as a-Si(n.sup.+)) 16, signal bus-line 18, source electrode 18' connecting the a-Si(n.sup.+) TFT and the pixel electrode. In FIG. 2, boundary line 1.sub.1 indicates the boundary between the regions undergoing and not undergoing the anodic oxidation. With reference to the boundary line 1.sub.1, the right area is a region undergoing anodic oxidation, while the left area is a region not undergoing the same.
Further, a structure as shown in FIG. 32(a), (b) has been used near the gate electrode of a conventional TFT substrate. FIG. 32(a) is a plan view near the gate electrode and FIG. 32(b) is a cross sectional view taken along line AA' thereof. In the figure, are shown substrate 10, Cr 11, A1 12', SiN 15, a-Si 15', source electrode 55, signal bus-line also serving as the drain electrode 18 and transparent electrode 17 as a pixel electrode.
As shown in the figure, Cr has been used for the gate electrode and SiN has been used for the gate insulator film. On the other hand, two layers of metals of Cr and Al are used for the gate bus-line. The reason why the gate electrode and the gate bus-line are constituted with different materials will now be described below.
At first, the conditions for the metal of the gate electrode are that it has good adhesion with the substrate, has no unevenness at the surface and does not suffer from deterioration in the course of forming SiN as the gate insulation film. Cr is suitable for the conditions. On the other hand, it is demanded for the gate bus-line that the resistance is low. Since Cr has a intrinsic resistivity higher by one or more of orders as compared with Al, it is not suitable to the gate bus-line. On the other hand, A is liable to cause hillocks, tending to cause acicularly protruded defects at the surface. Further, since there is a problem that the hillocks generate in the step of forming SiN as the gate insulation film (usually deposited by means of a plasma CVD method at a substrate temperature of 200.degree. to 350.degree. C.), it can not be used for the gate electrode. Accordingly, Cr have been used for the gate electrode and metals of two layer structure comprising Cr and Al have been used for the gate bus-line.
On the other hand, there is Ta or Al anodic oxidation technology in the prior art (refer, for example, to Manual of Electrochemistry (Maruzen), p 874-892, published December, 1964). This is a technic of electrochemically oxidizing the surface of metal which has been used for the capacitor or surface coating.
The merit of the oxide film (insulator film) formed by this technic is that it less causes defects due to foreign particles. Accordingly, there has been a prior art of utilizing the above-mentioned technology to TFT (Japanese Patent Laid-Open Sho 58-147069 and 61-133662).
As the prior art relevant to the present invention, there can be mentioned Japanese Patent Laid-Open Sho 63-164 regarding anodic oxidation and Japanese Patent Laid-Open Sho 58-90770 and 58-93092 regarding a storage capacitance.
However, since Al is used for the gate terminal or the gate electrode and only the portion thereof is used by anodic oxidation in the prior art, there are the following problems.
(1) Al is used also to the gate electrode in the conventional TFT substrate as shown in FIG. 2. Usually, the gate terminal of the TFT substrate is used in a state being exposed to atmospheric air. Al is liable to be denatured such by electric-erosion and the use of Al for the gate terminal deteriorates the reliability of the TFT panel.
(2) Since Al results in rod-like crystals referred to as whiskers and hillocks due to thermal stresses and thus brings about surface unevenness, it is not desirable. In particular, whiskers are whisker-like defects each of several tens of micronmeter, which cause, for example, shorts between electrodes.
In this way, the gate terminal in the prior art described above involves a problems in view of the reliability of the gate terminal or the yield upon manufacture due to the occurrence of defects or the like.
Further, (3) the gate bus-line has to be electrically connected with an external circuit at the end thereof. Accordingly, it is necessary for such a device as not anodically oxidizing the portion. It may be considered to cover the portion with a resist so that it may be kept from direct contact with an anodic oxidation solution. However, there is a problem that Al is disconnected along the end of the resist due to the phenomenon attributable to electrostatic breakdown of the resist.
(4) In a case of using a positive type photoresist as a mask for anodic oxidation, there has been a problem that defects such as dissolving of Al are caused at the intersection between the Al pattern and the mask pattern for anodic oxidation.
(5) It is desirable that the film thickness of Al.sub.2 O.sub.3 is as thin as possible in view of the mutual conductance gm of TFT. On the other hand, an increased thickness is desired in view of the breakdown voltage. There has been a problem that the film thickness is not optimized.